The dispensing of liquid reagent droplets of small volume with high accuracy represents an essential process step in the production of medical diagnostic assays. These assays are often based on two-dimensional arrays of open wells, such as microtiter plates. Some examples of such dispensing systems are described in the following references; U.S. Pat. No. 4,107,658, U.S. Pat. No. 4,196,615, U.S. Pat. No. 4,417,473, U.S. Pat. No. 4,818,492, U.S. Pat. No. 5,304,347, U.S. Pat. No. 5,601,980, U.S. Pat. No. 6,029,896, U.S. Pat. No. 6,148,666, U.S. Pat. No. 6,213,354, U.S. Pat. No. 6,551,558, U.S. Pat. No. 6,823,730, U.S. Pat. No. 6,851,778, U.S. Pat. No. 6,875,404, US 2001/0016177 A1, WO 98/09151, WO 00/51736, WO 01/89694 A1, WO 02/26499 A1, WO 03/106936, EP 0,164,679, EP 0,355,791, EP 0,505,004, EP 0,725,267, JP 2004251818 A, and JP 2006058188 A.
However, the accurate determination of the volume for individual dispensed droplets still remains a problem.
It has been found that, in needle-based dispensing systems, the target amount of liquid that is leaving the inner needle space is in many cases very well controlled, e.g., by the specific motion of a dispensing piston. The amount of liquid that is actually reaching the receiving well may show irregularities, however, because part of the liquid leaving the inner needle space is creeping along the outer diameter of the dispense needle, and is thereby forming an amount of liquid that is “lost” in the particular dispensing act. This mechanism may repeat itself in one or more successive dispensing acts and a substantial amount of liquid may accumulate, therefore, on the outer needle diameter. Once a critical amount of liquid has been accumulated, this liquid will “join” a dispensed droplet, generating an actual dispensed volume that by far exceeds the target dispense volume.
Therefore an apparatus and method is required for an accurate determination of the actual dispensed volume for individual dispensed droplets.